Method for determining a rotor position of an electrical generator in a wind turbine

ABSTRACT

A method for determining a rotor position of an electrical generator in a wind turbine is described comprising determining a voltage of the electrical generator, determining a rotor position angle estimate based on the voltage of the electrical generator, determining a subsequent rotor position angle estimate through a feedback loop, based on a combination of the voltage of the electrical generator and the rotor position angle estimate. Further, a method to real time track encoder health is described comprising determining the phase angle of a reference voltage, determining the angle difference between the rotor position and the reference voltage, and determining the differentiation of the angle difference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/218,442, filed Jun. 19, 2009, and claims priority under 35 U.S.C.§119 to Danish Patent Application No. PA 2009-00754, filed Jun. 19,2009. The content of each of these applications is hereby incorporatedby reference herein in its entirety for all purposes.

TECHNICAL FIELD

The present invention relates generally to a method for determining arotor position of a synchronous electrical generator in a wind turbine.

BACKGROUND

In recent years, it has become very common to use wind for thegeneration of electrical power. In order to do this, wind is captured bya set of blades of a wind power plant. The captured wind causes a shaftconnected to the set of blades to rotate. The shaft is typicallyconnected to a rotor of an electrical generator which is rotated inaccordance with the rotation of the shaft, possibly at a multiple of therotation speed of the shaft in case the rotor is connected to the shaftvia a gearbox. The electrical generator converts the mechanical powerprovided by the wind in form of the rotation into electrical power whichmay be supplied to a power grid.

For various functions of a wind turbine generator, a determination ofthe rotation speed and/or the position of the rotor of the electricalgenerator is required, such as for stator flux control which allowscontrolling the magnitude of the electromagnetic power provided by theelectrical generator.

The determination of the rotation speed and the position of the rotor ofthe electrical generator is typically carried out by a so-called encoderwhich may have an offset, e.g., determines the angular position of therotor with a certain offset. Therefore, before any function for which adetermination of the rotation speed and/or the position of the rotor isrequired can be started, which typically includes the start of actualpower supply of the electrical generator to a power grid, offset encodercalibration has to be carried out, i.e. the encoder offset has to bedetermined by which the phase angle of the rotor output by the encoderis adjusted such that the adjusted phase angle is aligned with theactual phase angle of the rotor (e.g., a permanent magnet rotor).

A known method for encoder offset calibration from a single stator linevoltage is based on zero crossing detection of the line voltages outputby the electrical generator (i.e. a stator line voltage of theelectrical generator). At each cycle when the line voltage is found tohave changed from a negative value to a positive value, a zero crossingpoint is detected. At this time instant, the corresponding output by theencoder is captured and the offset is calculated. Similarly, a rotorflux may be computed based on peak value detection of a line voltagesignal.

Since the zero crossing detection is prone to the noise, the accuratedetection of the zero crossings for encoder offset calibration may bevery difficult to achieve, even after some measures are used to improveit like linear interpolation, averaging etc.

Another issue is the calibration time. Since only one zero crossingpoint in one cycle (of the line voltage) is used for calibration, theprocessing time is long.

Hence, an objective of the present invention may be seen in providing amethod for determining a rotor position of an electrical generator whichmay be used for encoder offset calibration and which is more accurateand requires less processing time.

Since the encoder is prone to failure in a WTG (wind turbine generator),a real time tracking of encoder health is required for reliableoperation. It should be able to track encoder health real time in normaloperation, and send the alarm in case of encoder failure andmalfunction. An object of the present invention may also be seen inproviding a method for this real time health tracking.

SUMMARY

According to one embodiment, a method for determining a rotor positionof an electrical generator in a wind turbine is provided, comprisingdetermining a voltage of the electrical generator; determining a rotorposition angle estimate based on the voltage of the electricalgenerator; and determining a subsequent rotor position angle estimatethrough a feedback loop, based on a combination of the voltage of theelectrical generator and the rotor position angle estimate.

According to one embodiment, the combination includes a coordinatetransformation of the voltage into a rotational frame using the rotorposition angle estimate.

According to one embodiment, the combination includes the determinationof the difference between the phase of the voltage and an expected phaseof the voltage, wherein the expected phase of the voltage is a phasethat is to be expected when the rotor of the electrical generator hasthe rotor position according to the rotor position angle estimate.

According to one embodiment, the rotor position angle estimate isdetermined based on the phase of the voltage of the electricalgenerator.

According to one embodiment, a voltage of the electrical generator at afirst time instant and a voltage of the electrical generator at a secondtime instant are generated, wherein the rotor position angle estimate isdetermined based on the voltage of the electrical generator at the firsttime instant, wherein the rotor position angle estimate is combined withthe voltage of the electrical generator at the second time instant; andwherein the subsequent rotor position angle estimate is determined basedon the combination.

According to one embodiment, the subsequent rotor position angleestimate is determined using a control loop.

According to one embodiment, the subsequent rotor position angleestimate is determined using a phase locked loop.

According to one embodiment, the subsequent rotor position angleestimate is determined using a PI controller.

According to one embodiment, the combination includes the determinationof the difference between the phase of the voltage and an expected phaseof the voltage, wherein the expected phase of the voltage is a phasethat is to be expected when the rotor of the electrical generator hasthe rotor position according to the rotor position angle estimate, andthe difference is used as input for the PI controller.

According to one embodiment, the PI controller is used to determine anangular speed of the voltage.

According to one embodiment, an integrator is used to integrate theangular speed of the voltage for rotor position estimation.

According to one embodiment, the subsequent rotor position estimate isdetermined based on an adjustable gain function which adjusts the rotorposition estimation speed.

According to one embodiment, the voltage is a stator voltage of theelectrical generator.

According to one embodiment, the voltage is determined from aline-to-line voltage (phase-to-phase voltage) of the electricalgenerator.

According to one embodiment, the phase voltages of the electricalgenerator are determined from the line-to-line voltage and the voltageis determined from the phase voltages.

According to one embodiment, the method is being carried out in a powergeneration system.

According to one embodiment, the power generation system comprises anencoder for a rotor position determination of the electrical generatorand the method further comprises determining an offset of the encoderbased on the subsequent rotor position angle estimate.

According to one embodiment, the encoder is calibrated based on theoffset.

According to one embodiment, the power generation system comprises anencoder for a rotor position determination of the electrical generatorand the method further comprises detecting whether a malfunction of theencoder has occurred based on the subsequent rotor position angleestimate.

According to one embodiment, the power generation system comprises anencoder for a rotor position determination of the electrical generator,and the method further comprises the determination of a rotor fluxamplitude of the electrical generator based on the voltage and an outputof the encoder.

According to another embodiment, the encoder output is an angular speedmeasurement of the electrical generator.

According to an embodiment, the subsequent rotor position estimate isused in the control of the electrical generator.

According to another embodiment, the subsequent rotor position estimateand the rotor flux amplitude are used in the control of the electricalgenerator.

According one embodiment, a method for determining a rotor position ofan electrical generator is provided, the method comprising determining avoltage of the electrical generator, determining a rotor position angleestimate based on the voltage of the electrical generator, anddetermining a subsequent rotor position angle estimate through afeedback loop, based on a combination of the voltage of the electricalgenerator and the rotor position angle estimate.

According to an embodiment, a computer readable medium having a computerprogram recorded thereon, the computer program including instructionswhich, when executed by a processor, make the processor perform a methodfor determining a rotor position of an electrical generator as describedabove is provided.

According to another embodiment, an apparatus for determining a rotorposition of an electrical generator in a wind turbine according to themethod described above is provided.

According to one embodiment, a method for reliable real time tracking ofencoder health in a wind turbine generator is provided comprisinginputting a voltage reference from a generator controller to a phaselock loop to obtain the angle of the voltage reference; computing thedifferentiation of the angle difference between the voltage referenceangle and the calibrated rotor angle; sending, if the angle differenceis more than a threshold, an encoder failure signal to a systemsupervision configured to take necessary action and start ramping downthe turbine power.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views.

The drawings are not necessarily to scale, emphasis instead generallybeing placed upon illustrating the principles of the invention. In thefollowing description, various embodiments of the invention aredescribed with reference to the following drawings, in which:

FIG. 1 illustrates a common setup of a conventional wind turbine.

FIG. 2 illustrates an example of power generation system according to anembodiment of the present invention.

FIG. 3 illustrates a vector diagram for a synchronous electricalgenerator represented in a stationary reference frame.

FIG. 4 shows a block diagram for determining a rotor position of anelectrical generator according to an embodiment of the presentinvention.

FIG. 5 shows a flow diagram according to an embodiment of the presentinvention.

FIG. 6 shows a block diagram for encoder offset calibration according toan embodiment of the present invention.

FIG. 7 shows a graph shows the output of a function F(d,q) versus thephase error (i.e., the graph the function F(d,q)) according to oneembodiment.

FIG. 8 shows a block diagram for real time tracking of encoder healthaccording to an embodiment of the present invention.

FIG. 9 shows a block diagram according to an embodiment.

DETAILED DESCRIPTION

Exemplary embodiments of the method for controlling an electricalgenerator in accordance with the present invention will be described indetail below with reference to the accompanying figures. It will beappreciated that the exemplary embodiments described below can bemodified in various aspects without changing the essence of theinvention. Furthermore, embodiments described in the context of themethod for determining a rotor position of an electrical generator in awind turbine are analogously valid for the apparatus and the computerreadable medium.

FIG. 1 illustrates a common setup of a conventional wind turbine 100.The wind turbine 100 is mounted on a base 102. The wind turbine 100includes a tower 104 having a number of towers sections, such as towerrings. A wind turbine nacelle 106 is placed on top of the tower 104. Thewind turbine rotor includes a hub 108 and at least one rotor blade 110,e.g. three rotor blades 110. The rotor blades 110 are connected to thehub 108 which in turn is connected to the nacelle 106 through a lowspeed shaft which extends out of the front of the nacelle 106.

FIG. 2 illustrates an example of power generation system 200 accordingto an embodiment.

A shaft 10 transfers mechanical energy from an energy source, forexample the at least one rotor blade 110 shown in FIG. 1, to a rotor ofa variable speed generator 11. The shaft 10 is connected to the at leastone rotor blade 11 and is for example connected to the rotor via agearbox in order to adapt the rotational speed of the shaft 10 (i.e. thespeed of the wind turbine blades) to a speed range suitable for thegenerator 11. The generator 11 converts the mechanical energy providedvia the shaft 10 into electrical energy and delivers the electricalenergy at a set of stator terminals 12 a, 12 b, 12 c. The rotationalspeed of the shaft 10 varies as a function of the wind speed. Since therotational speed of the rotor of the generator 11 is proportional to therotational speed of the shaft 10, the amplitude and frequency of thevoltage signal provided by the generator 11 at the stator terminals 12a, 12 b, 12 c varies in accordance with the rotational speed of theshaft 10. The generator may be a synchronous generator, e.g. a permanentmagnet (PM) generator or any other type of generator comprising a statorwinding. The terminals 12 a, 12 b, 12 c of the generator 11 areconnected to a generator side power converter 13. The converter 13 forexample comprises a set of switches in the form of, for example,MOSFETs, GTOs, IGBTs or BJTs.

The converter 13 functions, under normal operation, as an activerectifier converting the variable frequency AC voltage provided by thegenerator 11 into a DC voltage. The conversion may be controlled using apulse width modulation (PWM) scheme, wherein control signals are appliedto the switches of the converter 13 in order to provide the desiredconversion functionality. In one embodiment, the switches are controlledby employing a space vector modulation scheme.

The output of the converter 13 is provided to a DC link 14 whichcomprises a link capacitor for reducing the voltage ripple on the DClink.

The DC link 14 is connected to a grid side power converter 15. Thetopology of the grid side power converter 15 may be similar to thegenerator side power converter 13. The grid side power converter 15 forexample normally operates as an inverter for converting the DC voltageon the DC link 14 into a regulated AC voltage for feeding active andreactive power to a power grid 18.

The output of the grid side power converter 15 may be filtered by meansof inductors 16 a, 16 b, and 16 c in order to, for example, remove highorder harmonics from the output power signal. The output power signal isthen provided to the power grid 18 via a transformer 19. The outputpower signal may, if needed, be filtered by a filter 17 in order to keepthe interference or harmonic distortion at a low value.

FIG. 3 illustrates a vector diagram 300 for a synchronous electricalgenerator represented in a stationary reference frame.

The diagram comprises two stationary axes denoted α and β. Thestationary reference frame is thus also referred to as αβ (reference)frame.

A transformation of a voltage from the three phase stationary coordinatesystem, which may also be referred to as the stationary three phase abcreference frame, to the αβ frame may be performed according to

$\begin{bmatrix}U_{\alpha} \\U_{\beta}\end{bmatrix} = {\begin{bmatrix}{2/3} & {{- 1}/3} & {{- 1}/3} \\0 & {\sqrt{3}/3} & {{- \sqrt{3}}/3}\end{bmatrix} \cdot \begin{bmatrix}U_{a} \\U_{b} \\U_{c}\end{bmatrix}}$

wherein U_(a), U_(b), U_(c) refer to the three phase voltages and U_(α),U_(β) refer to the components of the voltage in the αβ frame.

From the αβ stationary coordinate system, i.e. the αβ frame, atransformation may be performed into a dq rotating coordinate systemaccording to

$\begin{bmatrix}U_{q} \\{Ud}\end{bmatrix} = {\begin{bmatrix}{{- \sin}\; \theta} & {\cos \; \theta} \\{\cos \; \theta} & {\sin \; \theta}\end{bmatrix} \cdot \begin{bmatrix}U_{\alpha} \\U_{\beta}\end{bmatrix}}$

where θ=ωt is the angle between the stationary a axis and thesynchronous d axis, i.e. the axis synchronous with the rotor.

The voltages U_(α), U_(β) may be normalized to U_(α) _(—) _(nom), U_(β)_(—) _(nom) in accordance with

$\begin{bmatrix}U_{\alpha\_ nom} \\U_{\beta\_ nom}\end{bmatrix} = {\frac{1}{\sqrt{U_{\alpha}^{2} + U_{\beta}^{2}}} \cdot {\begin{bmatrix}U_{\alpha} \\U_{\beta}\end{bmatrix}.}}$

In FIG. 3, a first vector, denoted by Ψ_(mag), represents themagnetizing flux.

In the example shown in FIG. 3, which refers to a synchronous generator,the magnetizing flux corresponds to the rotor flux Ψ_(r). The rotor fluxmay be generated by means of a permanent magnet, as in a PM generator,by excitation of a field coil in the rotor (i.e. a wound generator). Thearc at the tip of the rotor flux vector illustrates that the vectorrotates about the origin of coordinates in FIG. 3. The angulardisplacement of the rotor flux vector from the α axis is denoted byθ_(r) in FIG. 3.

In a corresponding manner, the stator flux vector, denoted by Ψ_(S) inFIG. 3, is a vector which rotates about the origin of coordinates.

In steady state operation the stator flux vector rotates in thestationary reference frame with an angular speed equal to the rotor fluxvector. The angular displacement of the stator flux vector from therotor flux vector is denoted by δ in FIG. 3.

The electromagnetic power P_(EM) of a synchronous generator isproportional to ωΨ_(S)×Ψ_(r) where ω is the rotational speed of therotor. This means that

P _(EM) =f(|Ψs|,|Ψr|,δ).

From this, it can be seen that for a given speed of operation (i.e. agiven rotor rotation speed), the electromagnetic power depends on themagnitude of the stator flux vector and its location with respect to therotor flux vector. If the position of the rotor flux vector is known, itis possible to apply a voltage to the stator that positions the statorflux vector to give the desired magnitude of the power at a givenrotational speed. Hence, by controlling the stator flux vector, theelectromagnetic power, which corresponds to the power given to the load,can be obtained as desired.

The stator flux vector may for example be controlled by a suitablecontrol of the generator side power converter 13. Accordingly, in oneembodiment, the power generation system 200 includes a stator fluxcontroller 20 which controls the generator side power converter 13 suchthat the power supplied by the electrical generator 11 to the power grid18 has a desired magnitude. The stator flux controller 20 needs, in oneembodiment, information about the rotor position, e.g. a phase angle ofthe rotor, and/or information about the angular speed of the rotor. Thisinformation is for example supplied by an encoder 21 which for examplegenerates the information about the rotor position and/or the angularspeed based on measurements.

The information output by the encoder 21 may also be used for otherfunctions than the stator flux control.

The encoder 21 may have an offset, e.g. the angular position determinedby the encoder may differ from the actual angular position of the rotorby a certain offset. Therefore, in one embodiment, before any functionfor which a determination of the rotation speed and/or the position ofthe rotor is required can be started, which may include the start ofpower generation, i.e. actual power supply of the electrical generator11 to the power grid 18, an offset encoder calibration is carried out,i.e. an encoder offset is determined by which the phase angle of therotor output by the encoder is adjusted such that the adjusted phaseangle is aligned with the actual phase angle of the rotor (e.g. apermanent magnet rotor).

According to one embodiment, a block diagram for determining a rotorposition of an electrical generator is provided as illustrated in FIG.4, which may, for example, be used for encoder offset calibration.

FIG. 4 shows a block diagram 400 for determining a rotor position of anelectrical generator according to an embodiment.

The block diagram 400 may include a voltage determining block 401configured to determine a voltage 402 of the electrical generator.Further, the block diagram 400 includes an angle estimating block 403configured to determine a rotor position angle estimate 404 based on thevoltage 402 of the electrical generator.

The block diagram 400 further includes a feedback block 405 configuredto combine the rotor position angle estimate 404 with the voltage of theelectrical generator. The angle estimating block 403 is configured todetermine a subsequent rotor position angle estimate 404 based on thecombination.

The block diagram 400 may for example be implemented by a processorwhich is programmed in accordance with the functionality of the blockdiagram 400 using software. The software may for example be executed bya processor in a power controller of the wind turbine. The block diagram400 may also be implemented using a circuit. In an embodiment, a“circuit” may be understood as any kind of a logic implementing entity,which may be special purpose circuitry or a processor executing softwarestored in a memory, firmware, or any combination thereof. Thus, in anembodiment, a “circuit” may be a hard-wired logic circuit or aprogrammable logic circuit such as a programmable processor, e.g. amicroprocessor (e.g. a Complex Instruction Set Computer (CISC) processoror a Reduced Instruction Set Computer (RISC) processor). A “circuit” mayalso be a processor executing software, e.g. any kind of computerprogram, e.g. a computer program using a virtual machine code such ase.g. Java. Any other kind of implementation of the respective functionswhich will be described in more detail below may also be understood as a“circuit” in accordance with an alternative embodiment.

In other words, in one embodiment, a control loop is used fordetermining the rotor position angle based on the voltage vector (or thevoltage vector angle). The estimate of the rotor position angle may beseen as the controlled variable in one embodiment and may be seen to becontrolled such that it corresponds to the phase angle of the currentvoltage vector, i.e. has the value that it should theoretically havebased on the phase angle of the current voltage vector, e.g. Pi/2 lessthan the phase angle of the current voltage vector. In one embodiment,the voltage angle is subtracted by pi/2 to obtain the rotor angleposition because the voltage angle leads the rotor angle by pi/2.

In one embodiment, the combining includes a coordinate transformation ofthe voltage in accordance with the rotor position angle estimate, forexample a coordinate transformation of the voltage to a coordinatesystem fixed with the rotor position as specified by the rotor positionangle estimate.

According to one embodiment, the combining includes the determination ofthe difference between the phase of the voltage and a phase of thevoltage to be expected when the rotor of the electrical generator hasthe rotor position according to the rotor position angle estimate.

The angle estimating block 403 may be configured to determine the rotorposition angle estimate based on the phase of the voltage of theelectrical generator.

In one embodiment, the voltage determining block 401 is configured todetermine a voltage of the electrical generator at a first time instantand a voltage of the electrical generator at a second time instant,wherein the angle estimating block 403 is configured to determine therotor position angle estimate based on the voltage of the electricalgenerator at the first time instant, wherein the feedback block 405configured to combine the rotor position angle estimate with the voltageof the electrical generator at the second time instant; and wherein theangle estimating block 403 is configured to determine the subsequentrotor position angle estimate based on the combination.

According to one embodiment, the angle estimating block 403 and thefeedback block 405 are configured according to a phase locked loop forrotor position angle determination.

In one embodiment, the angle estimating block 403 and the feedback block405 form a control loop.

The angle estimating unit may for example include a PI controller.

In one embodiment, the combining includes the determination of thedifference between the phase of the voltage and a phase of the voltageto be expected when the rotor of the electrical generator has the rotorposition according to the rotor position angle estimate and the PIcontroller receives the difference as input.

For example, the PI controller is configured to determine an angularspeed of the voltage determined by the voltage determining block 401.

The angle estimator block 403 may include an integrator configured tointegrate the angular speed of the voltage for rotor positionestimation.

In one embodiment, the feedback block 405 is configured to determine amapping function which adjusts the rotor position estimation speed andthe angle estimating block 403 is configured to determine the subsequentrotor position estimate based on the output of the mapping function.

In one embodiment, the voltage is a stator voltage of the electricalgenerator.

In one embodiment, the voltage determining block 401 is configured todetermine the voltage from a line-to-line voltage of the electricalgenerator.

For example, the voltage determining block 401 is configured todetermine the phase voltages of the electrical generator (e.g. the threephase voltages when the electrical generator is a generator with threephases) from the line-to-line voltage and to determine the voltage fromthe phase voltages.

According to one embodiment, a power generation system is providedcomprising the block as described above. The power generation system mayfurther comprise an encoder for a rotor position determination of theelectrical generator and may further comprise an encoder offsetcalibration block determining an offset of the encoder based on thesecond rotor position angle estimate. For example, the encoder offsetcalibration block is configured to calibrate the offset of the encoder.

The power generation system may also comprise an encoder for a rotorposition determination of the electrical generator and further comprisea detection block configured to detect a malfunction of the encoderbased on the second rotor position angle estimate. For example, therotor position estimation may be used for real time tracking thefunctionality of the encoder.

The electrical generator is for example a wind turbine generator.

The block diagram 400 described above with reference to FIG. 4 mayfurther include encoder offset calculation and may be realized using aPLL. This is described in the following with reference to FIG. 9.

FIG. 9 shows a block diagram 900 according to an embodiment.

The block diagram 900 includes an OTT block 901 which receives aphase-to-phase voltage U_(ab) as input and generates three phasevoltages U_(abc). The phase voltages U_(abc) are fed to a PLL 902 whichgenerates an estimated voltage phase angle γ_(est). The PLL 902 furtheroutputs an amplitude of a voltage vector |U_(S)| which is fed to a rotorflux calculation block 903 which also receives an angular speed ω_(e) asinput and generates a rotor flux amplitude Ψ_(r).

The estimated voltage phase angle γ_(est) and an angular rotor positionas output by the encoder γ_(e) are fed to an offset calculation block904 which determines the encoder offset.

The block diagram 900 described above with reference to FIG. 9 will bedescribed later in more details with reference to FIG. 6.

The block diagram 400 for example carries out a method for determining arotor position of an electrical generator as illustrated in FIG. 5.

FIG. 5 shows a flow diagram according to an embodiment.

In 501, a voltage of the electrical generator is determined.

In 502, a rotor position angle estimate is determined based on thevoltage of the electrical generator.

In 503, a subsequent rotor position angle estimate is determined basedon a combination of the rotor position angle estimate with the voltageof the electrical generator.

The method may for example be carried out by an apparatus which issuitably configured. In one embodiment, the method is carried out by aprocessor which is programmed to carry out the method.

According to one embodiment, the method may comprise

-   -   1. Determine the three phase stator voltages of the generator;    -   2. Determine the voltage angle;    -   3. Determine the rotor position based on voltage angle;    -   4. Calibrate encoder based on estimated rotor position from step        3 and encoder position output.

According to one embodiment, a robust and accurate solution for encoderposition offset calibration and rotor flux magnitude computation isprovided. According to another embodiment, a real time tracking ofencoder health for reliable operation is provided.

Embodiments may for example be applied to a permanent magnet (PM)generator of a wind turbine generator. As explained above, encodercalibration may typically be a pre-required step before any generatorcontrol task can proceed for a PM machine. In a PM machine, the encodercalibration may for example include the process to align the positionreading from the encoder with the location of the north pole of thepermanent magnet of the electrical generator. Further, since an encoderof a wind turbine generator is prone to failure, a real time tracking ofencoder health may be required for reliable operation. In oneembodiment, encoder health (e.g. whether the encoder functions correctlyor functions within a pre-defined accuracy range) is tracked in realtime in normal operation and an alarm is sent in case of encoder failureor malfunction.

An example for a block for encoder offset calibration according to oneembodiment which includes a block for determining a rotor position of anelectrical generator is explained in the following with reference toFIG. 6.

FIG. 6 shows a block diagram 600 for encoder offset calibrationaccording to an embodiment.

The method carried out by the apparatus 600 serves for encoder offsetcalibration and rotor flux calculation. By this method the encoderoffset is determined, so that by adjusting the encoder output with thisoffset, the rotor position is obtained.

As one input, the block diagram 600 receives a stator phase-to-phasevoltage U_(ab) of the electrical generator. The phase-to-phase voltageis fed to an OTT (One-To-Three technology) block 601 which contracts thethree phase voltages U_(a), U_(b), U_(c) from the single line voltageU_(ab) by using One-To-Three technology (OTT). In another embodiment,the three phase stator voltages U_(a), U_(b), U_(c) may also bedetermined by measurement of the three phase stator voltages.

The three phase voltages U_(a), U_(b), U_(c) are fed to a phase lockloop control block. By this phase lock loop, the phase angle of threephase voltage is obtained.

For this, the three phase voltages U_(a), U_(b), U_(c) are transformedto the αβ frame, i.e. to the stator α-axis component U_(α) and thestator β-axis component U_(β) in αβ frame by an abc/αβ transformingblock 602.

A normalization block 603 normalizes the stator α-axis component U_(α)and the stator β-axis component U_(β), e.g. in accordance to

${\begin{bmatrix}U_{\alpha\_ nom} \\U_{\beta\_ nom}\end{bmatrix} = {\frac{1}{\sqrt{U_{\alpha}^{2} + U_{\beta}^{2}}} \cdot \begin{bmatrix}U_{\alpha} \\U_{\beta}\end{bmatrix}}},$

i.e. by calculating the amplitude of voltage vector as

|U _(S)|=√{square root over ((U _(α))²+(U _(β))²)}{square root over ((U_(α))²+(U _(β))²)}.

The normalization process may be used to eliminate the effect of voltageamplitude in the phase lock loop control.

For determining the rotor flux amplitude, the amplitude of voltagevector |U_(S)| is fed to a divider which divides the amplitude ofvoltage vector |U_(S)| by the angular speed ω_(e) output by the encoder.The result of the division is output, possibly after filtering by afirst low pass filter (LPF) 605, as the rotor flux amplitude Ψ_(r). Inother words, the rotor flux amplitude Ψ_(r) is computed asΨ_(r)=|U_(S)|/ω_(e). The determined rotor flux may for example be usedfor rotor flux calibration. As indicated above, the rotor position andthe rotor flux amplitude may be used in the control of the electricalgenerator.

An αβ/qd transforming block 606 transforms the normalized stator α-axiscomponent U_(α) and the normalized stator β-axis component U_(β) to thedq frame using the current estimate for the voltage vector phase angleγ_(est), e.g. in accordance with

$\begin{bmatrix}U_{sq} \\{Usd}\end{bmatrix} = {\begin{bmatrix}{\cos \; \gamma_{est}} & {\sin \; \gamma_{est}} \\{\sin \; \gamma_{est}} & {{- \cos}\; \gamma_{est}}\end{bmatrix} \cdot {\begin{bmatrix}U_{\alpha} \\U_{\beta}\end{bmatrix}.}}$

where U_(sd) is the stator voltage d-axis component in d/q frame andU_(sq) is the stator voltage q-axis component in dq frame.

The αβ/qd transforming may be seen as transforming the voltage in α/βframe to dq frame using the feedback signal, i.e. the currentlyestimated voltage angle.

The stator voltage d-axis component in d/q frame U_(sd) and the statorvoltage q-axis component in d/q frame U_(sq) are fed to a mapping block607 which maps these components to a function value of a function Fwhich may be seen as function of the phase error Δθ between the actualvoltage phase angle (i.e. the phase angle of the current stator voltageas represented by U_(sd) and U_(sq)) and the estimated voltage vectorphase angle (i.e. the fed back estimate for the stator voltage phaseangle γ_(est)) according to

F(Δθ)=tan(Δθ)=Usd/Usq,|Δθ|<Δθ _(m)

F(Δθ)=−F _(Lim),−π<Δθ<−Δθ_(m)

F(Δθ)=F _(Lim),Δθ_(m)<Δθ<π

where Δθ_(m) is a pre-defined threshold value.

The phase error Δθ is for example determined according totan(Δθ)=Usd/Usq.

Mapping block 607 can be described as an adjustable gain functionF(d,q), which is used to adjust the tracking speed according to thetuning phase error. The function F is illustrated in FIG. 7.

FIG. 7 shows a graph shows the output of a function F(d,q) versus thephase error (i.e. the graph the function F(d,q)) according to oneembodiment.

A first axis 701 (x-axis) of the graph 700 corresponds to the possiblevalues of the phase error Δθ and a second axis 702 (y-axis) of the graph700 corresponds to the function values of F.

The output of the mapping block 607 is compared to zero (which may beseen as the should-be value of the phase error Δθ) by a first comparingblock 608. The difference between the phase error Δθ and zero is inputto a PI controller 609 with output limiting. The output of the PIcontroller is the angular speed of the voltage vector (as represented byU_(sd) and U_(sq)) in the dq frame.

From a signal processing point of view, the PI controller 609 may beseen to function as a low pass filter that filters the voltage noisesuch that a high angle estimation accuracy may be achieved.

A first adder 610 adds the angular speed ω_(e) from encoder output andoutput from PI controller to generate the estimated angular speedω_(est) of the voltage vector in αβ frame.

An integrator 611 generates the estimated phase angle of the voltagevector γ_(est) by integration of the angular speed in αβ frame.

In one embodiment, since the stator voltage phase (tracking) error canbe in range of −π to π, the function F provides a faster trackingmechanism in this range. When the angle difference is within −Δθ_(m) andΔθ_(m), a tangent (tan) function may be used and when the absolute valueof the phase angle error is larger than Δθ_(m), the output of themapping block 607 is clamped at max limit. Therefore, the output of thePI controller (the angular speed) will be adjusted according to theangle difference to eliminate the phase tracking error.

A first subtracter 612 subtracts π/2 from γ_(est) to generate the PMrotor phase angle corresponding to the estimated voltage phase angleγ_(est). A second comparing block 613 compares the PM rotor phase angleoutput by the first subtracter 612 with the angular rotor position asoutput by the encoder γ_(e). The output of the second comparing block613 is, possibly filtered by a second low pass filter 614, output as theencoder offset. The encoder offset may be output and may be used tocalibrate the encoder. The calibration may be done by adding thedetermined offset to the angular rotor position as output by the encoderγ_(e) by a second adder 615 and transforming the result to an anglewithin −π and π by a transforming block 616 to generate a calibratedangular position γ_(cal).

As explained above, the encoder calibration may be necessary to becarried out before normal operation mode, i.e. before power is actuallysupplied from the electrical generator 11 to the power grid 18.

In normal operation mode (in other words, during power generation, i.e.supply of power to the power grid), a similar method as described in 600may be used for the real time tracking of encoder health.

FIG. 8 shows a block diagram 800 for real time tracking of encoderhealth according to an embodiment.

The block diagram 800 may be seen as being identical to block diagram600 shown in FIG. 6 except for a minor modification. The block diagram800 includes a generator controller 801 which outputs a referencevoltage in α/β frane −U_(α) and U_(β). The block diagram 800 furtherincludes, analogously to the block diagram 600, a normalization block803, an αβ/qd transforming block 806, a mapping block 807, a firstcomparing block 808, a PI controller 809, a first adder 810, and anintegrator 811 which serve to calculate an estimate of the phase angleγ_(est) of the voltage input by the generator controller 801 as it hasbeen explained above in the context of FIG. 6.

A second adder 804 of the block diagram 800 adds the encoded offsetdetermined previously, e.g. by the block diagram 600 before powergeneration mode has been started, to the angular rotor position asoutput by the encoder to generate the calibrated angular positionγ_(cal).

During power generation, in one embodiment, a control system of thepower generation system ensures that U_(α) and U_(β) change smoothly.

Therefore, the difference between the calibrated encoder angularposition γ_(cal) and the estimated angular position of the referencevoltage γ_(est) also changes smoothly. This difference is calculated bya subtracter 805 and the result is differentiated by a differentiator812. The result of the differentiation γ_(diff) of the (γ_(cal)−γ_(est))signal may be expected to be close to a constant in generation operationdynamic and close to zero at steady state. However, an encoder failureor malfunction may be expected to cause γ_(diff) to become very large.Thus, according to one embodiment, by continuously monitoring theγ_(diff) signal, encoder failure or encoder malfunction may be detected.

According to one embodiment, a method and a system for encodercalibration is provided that is less sensitive to measurement noise andmotor speed variation and allows calibrating an encoder with highaccuracy. According to one embodiment, only one voltage sensor isrequired for calibration, e.g. one voltage sensor for measuring oneline-to-line stator voltage.

Further, fast calibration convergence and short calibration time at alloperation speed levels may be achieved. Additionally, embodiments allowreal time tracking of encoder health for reliable operation.

According to an embodiment, a method for determining a rotor position ofan electrical generator is provided. This method includes the followingsub method:

-   -   1) Determine 3 phase voltage based on one phase-to-phase        voltage. (OTT)    -   2) A method to track the phase angle of 3 phase voltage (PLL)        -   a. PI controller is used        -   b. An integration is used to obtain the angle from angular            speed        -   c. An adjustable gain function F(d,q) is used to adjust the            tracking speed according to the tuning phase error.        -   d. The estimated voltage angle is the feedback signal in PLL            control        -   e. A dq transformation is used        -   f. A α/β transformation is used    -   3) Calculate the encoder offset based on output from 2) and        encoder position output    -   4) The rotor position is obtained by using encoder position        output and the offset obtained from above.    -   5) Calculate the rotor flux based on output from 2) and encoder        angular speed output

According to an embodiment, a method for real time encoder healthtracking is provided. This method includes the following sub function;

-   -   1) Determine the phase angle of the reference voltage from        generator controller using PLL    -   2) Compare this with the calibrated rotor phase angle.    -   3) By a differentiation process, the encoder health is monitored        real time.

According to an embodiment, a method for determining a rotor position ofa synchronous electrical generator in a wind turbine is providedcomprising determining voltage of the electrical generator; determininga rotor position angle estimate based on the back Emf(electromotiveforce) voltage of the electrical generator; obtaining a rotor positionangle determination from an encoder arranged on the electricalgenerator; combining the rotor position angle estimate and the rotorposition angle from encoder output to obtain an offset value of theencoder the offset value being used to determine a calibrated rotorposition; wherein the rotor position angle estimate is determined bypassing the voltage of the electrical generator through a Phase lockloop control method.

In one embodiment, the voltage is a phase-to-phase voltage and themethod further comprises converting the phase-to-phase voltage to threephase voltages of the electrical generator.

In one embodiment, a phase lock loop (PLL) is used to obtain the voltagephase angle from the three phase voltages. In one embodiment, when aphase lock loop is used, the voltage vector amplitude is obtained. Bydividing the vector amplitude by the rotor speed, the rotor flux isobtained.

In one embodiment, the offset is obtained by subtracting the voltagephase angle by pi/2. The encoder offset is for example obtained bycombining this rotor angle with the rotor angle from the encoder.

In one embodiment, the rotor flux amplitude Ψr is obtained as aby-process.

While embodiments of the invention have been particularly shown anddescribed with reference to specific embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention as defined by the appended claims. The scope of theinvention is thus indicated by the appended claims and all changes whichcome within the meaning and range of equivalency of the claims aretherefore intended to be embraced.

1. A method for determining a rotor position of an electrical generatorin a wind turbine, the method comprising: determining a voltage of theelectrical generator; determining a rotor position angle estimate basedon the voltage of the electrical generator; and determining a subsequentrotor position angle estimate through a feedback loop, based on acombination of the voltage of the electrical generator and the rotorposition angle estimate, wherein the subsequent rotor position angleestimate is determined based on an adjustable gain function whichadjusts the rotor position estimation speed.
 2. The method of claim 1,wherein the voltage is a stator voltage of the electrical generator. 3.The method of claim 2, wherein the voltage is determined from aline-to-line voltage of the electrical generator.
 4. The method of claim3, wherein the phase voltages of the electrical generator are determinedfrom the line-to-line voltage and the voltage is determined from thephase voltages.
 5. The method of claim 1, being carried out in a powergeneration system.
 6. The method of claim 5, wherein the powergeneration system comprises an encoder for a rotor positiondetermination of the electrical generator and the method furthercomprises determining an offset of the encoder based on the subsequentrotor position angle estimate.
 7. The method of claim 6, wherein theencoder is calibrated based on the offset.
 8. The method of claim 5,wherein the power generation system comprises an encoder for a rotorposition determination of the electrical generator and the methodfurther comprises detecting whether a malfunction of the encoder hasoccurred based on the subsequent rotor position angle estimate.
 9. Themethod of claim 5, wherein the power generation system comprises anencoder for a rotor position determination of the electrical generator,and the method further comprises the determination of a rotor fluxamplitude of the electrical generator based on the voltage and an outputof the encoder.
 10. The method of claim 9, wherein the encoder output isan angular speed measurement of the electrical generator.
 11. The methodof claim 1, wherein the subsequent rotor position estimate is used inthe control of the electrical generator.
 12. The method of claim 9,wherein the subsequent rotor position estimate and the rotor fluxamplitude are used in the control of the electrical generator.
 13. Acomputer readable medium having a computer program recorded thereon, thecomputer program comprising instructions which, when executed by aprocessor, make the processor perform a method for determining a rotorposition of an electrical generator in a wind turbine, comprisingdetermining a voltage of the electrical generator; determining a rotorposition angle estimate based on the voltage of the electricalgenerator; and determining a subsequent rotor position angle estimatethrough a feedback loop, based on a combination of the voltage of theelectrical generator and the rotor position angle estimate, wherein thesubsequent rotor position angle estimate is determined based on anadjustable gain function which adjusts the rotor position estimationspeed.
 14. An apparatus for determining a rotor position of anelectrical generator in a wind turbine, the apparatus being configuredto determine a voltage of the electrical generator; determine a rotorposition angle estimate based on the voltage of the electricalgenerator; and determine a subsequent rotor position angle estimatethrough a feedback loop, based on a combination of the voltage of theelectrical generator and the rotor position angle estimate, wherein thesubsequent rotor position angle estimate is determined based on anadjustable gain function which adjusts the rotor position estimationspeed.